In industrial applications there is often a need to line certain elements with a material which exhibits properties quite different from the material from which the element is made. As an example may be mentioned tubes, tanks, nozzles or similar devices which are to withstand, for example, high temperature, variations in temperature, erosion from particle flows, or some other type of degrading influence on the material.
The present invention is applicable within each technical field in which linings of the above-mentioned kind are desirable. To describe the problems which arise in connection with lining and the solutions that are currently applied, known technique involving ceramic lining of tubes exposed to high temperatures and/or erosive particle flows is chosen.
In boiler plants, flue gas tubes, cleaning plants for flue gases and the like, usually some steel quality is used in walls and casings. If these casings were to be exposed to the influence of very hot flue gases containing erosive particles for a substantial period of time, and under the effect of temperature fluctuations, these casings would be worn out relatively quickly. For this reason, cyclones, for example, which constitute dust cleaning devices in, among other things, fossil fuelled power plants, are usually provided with a lining, usually of a ceramic material.
In a cyclone, a vortex of, for example, flue gases moves from a higher to a lower level inside a cyclone wall with a downwardly tapering circular cross section. During the downward flow, the speed of the gas flow increases, causing heavier particles present in the gas vortex to be thrown out against the cyclone wall and then to fall down into cyclone legs which form dust outlets from the cyclone. The cleaned gas is discharged at the upper part.
For operation of gas turbines connected to cyclones of the above-mentioned type, the highest possible gas temperature is desired at the inlet of the turbine. This means that gas cleaners which separate dust from combustion gases operate at the temperature of the combustion gases when these leave a bed in a combustion plant.
A combustion plant of, for example, PFBC type operates with a gas temperature which may amount to 950.degree. C. This high temperature entails heavy stresses in the cyclones for cleaning of the combustion gases before these are supplied to a turbine. The problems are particularly significant in the lowermost part of the cyclone and in the cyclone legs. The high speed of the greatly abrasive, erosive particles in the gas mass and the high temperature reduce the strength of the cyclone material and deteriorate its resistance to wear.
Despite different forms of cooling of cyclones and different designs of cyclones and cyclone legs, the problem with the heavy wear on the cyclone material from dust in the gas remains. This has made it necessary to provide cyclones with an erosion-resistant material, usually in the form of a lining. This lining may be formed from ceramic material, which has long been known in the art. In already existing PFBC energy plants, the cyclones have been internally lined with a high-resistant ceramic material.
One way of providing cyclones or other corresponding devices with a ceramically resistant material according to known technique is to apply a steel net with hexagonal meshes to the surface which is to be coated. The net is spot welded to this surface. The net has a certain thickness, since the net is formed from steel bands. Inside each mesh there are central holes in the steel band. After application, the meshes in the steel net are filled with a ceramic material, usually aluminium oxide, which is fixed in position by the ceramic material penetrating also the holes in the steel band. The ceramic gives the surface good resistance to erosion and provides good protection against fires which may arise under certain conditions. In addition, the ceramic withstands temporary increases in temperature. One problem, however, is caused by the different coefficients of linear expansion of the ceramic and the lined material.
Upon start-up, cyclones are heated from room temperature to operating temperature for a relatively long time. When the ceramic gradually reaches the operating temperature, the temperature of the cyclone wall has risen to about 850.degree. C. or around 350.degree. C. depending on whether insulation has been applied outside the cyclone wall or between the ceramic and the steel wall.
Prior to start-up of the plant, there are small gaps, at a temperature of about 20.degree. C., between the hexagonal ceramic plates inside the steel meshes and the steel bands of these meshes. During a heating period and because of the greater liner expansion of the steel material, the width of these gaps is increased, dust from the flue gases being packed into these gaps. During the subsequent shrinkage of the materials during an interruption of the operation, or a reduction of the temperatures of the two materials for some other reason, stresses in the ceramic material will arise because of the above-mentioned packing of dust into the gaps, which results in the ceramic being easily broken. This problem, of course, is aggrevated by repeated increases and falls in temperature.
Another problem arises with the existing temperature gradient across the inside and outside of the ceramic. Under certain conditions, the temperature difference beween inside and the outside of the ceramic is very great, which causes cracking of the ceramic material. One reason for these temperature differences is that flue gas is not allowed to sweep around the back of the ceramic material.
Nor are the currently used ceramics for the above-mentioned technique sufficiently erosion-resistant. More erosion-resistant ceramics are available but require a different application.
Another variant of the solution to the problem of two materials in a lining expanding to differing degrees during heating is taken from an example with a ceramic lining of a steel casing. Ceramic plates are provided with cast-in steel holders. These holders are welded to the steel casing such that a certain gap arises between the steel casing and the ceramic. The space formed by this gap is filled with insulation. In this way, the ceramic and the steel casing may be maintained at different temperatures. The two sides of the ceramic assume, for example, the temperature 850.degree. C. whereas the steel casing is allowed a maximum temperature of, for example, 350.degree. C. By controlling the temperature which is adopted by the respective material, it will be possible to impart to each material the same linear expansion. This causes the two materials to be expanded to the same degree, so there will be no mutual displacements between the two materials. However, also this solution has its disadvantages, since what is stated above only applies to the steady state. Under heating or cooling conditions, stresses may arise or a dust-accumulating growth of gaps may arise in the ceramic.
A specification of requirements may be drawn up for a lining, the aim of which is to reach a solution to the problems described above. It is, for example, desirable to have large, smooth, continuous surfaces while avoiding joints in the ceramic lining. Another desire is to have as few contact points as possible between the lining and the casing. In addition, it is advantageous to provide a gap between the lining and the casing. In this way, in the example using cyclones, a small amount of gas may sweep over the back side of the lining, causing the back side to adopt the same temperature as the front to avoid temperature gradients over the lining material. By means of such a gap, dust penetrating in between the lining and the casing may also be allowed to exit out of the gap.
Aimed at a solution to the above-mentioned problems, a new design relating to a ceramic lining has been developed. However, the principle of the solution is of such a general nature that it may be applied to a plurality of technical fields.